From: Jim Freeman ([email protected])
Date: Fri Dec 17 1999 - 15:44:30 PST
Hi Mark,
After I wrote the missive, I realozed that I mispoke. You are exactly correct.
Jim Freeman
Marc Humphreys wrote:
> My simulation show something quite different.
> The 100 ohm far end termination between the two differntial input
> is required at the far end of the line to generate the differential
> voltage at the receiver. The LVDS drivers are typically 2 to 4ma
> current sources, which subsequently generate the
> 200 to 400 mv voltage swings at the receiver when the current flows
> out one side through the resistor and back thru the other side. No
> terminator
> and my _P and _N signals stay pretty static at ~ 0.9v and 1.5vrespectively.
>
> Jim, maybe the spice model for your using receiver has a built in 100 ohm
> terminator.
>
> There are a number of ways to generate the required input
> voltages at the input of an LVDS receiver, hence the reason for
> asking the vendor, is this a true LVDS implementation. God knows there all
> kinds of ways of taking ECL, PECL and CML etc. signals and massaging them
> to meet the LVDS specs but its a far cry from the simplicity of a low
> current source
> single, a 100ohm diff pair set of traces, and a single far-end terminator,
> discrete or
> on chip terminator.
>
> My 2 cents.
>
> Marc
>
> My 2 cents.
>
> Marc
> > -----Original Message-----
> > From: Jim Freeman [SMTP:[email protected]]
> > Sent: Thursday, December 16, 1999 4:33 PM
> > To: [email protected]
> > Subject: Re: [SI-LIST] : Physcially-small far-end LVDS terminations?
> >
> > Hi Eric,
> > Another answer would be to ensure that the differential impedance of
> > the board between the differential pairs is 100 ohms. Then you wouldn't
> > need the resistors. I have performed Hspice sims on this and it works fine
> > if the stackup/trace spacing is correct.
> >
> > Jim Freeman
> >
> > Eric Goodill wrote:
> >
> > > Hi,
> > >
> > > I want to use the typical LVDS termination where you have two 50-ohm
> > resistors in series placed between the two lines of the LVDS diff. pair
> > and also have a cap at the junction of the resistors to ground. I have a
> > jillion LVDS pairs to terminate at a high-pin-count ASIC, and I'm looking
> > to squeeze more terminations per square inch. (There's no hope of putting
> > the terminations inside the ASIC at this point.) I'm hoping some company
> > makes a single three-pin part the has those two resistors and a cap even
> > if not specifically for termination use.
> > >
> > > How about any other ideas for even higher density?
> > >
> > > Regards, Eric
> > > --
> > > Eric Goodill
> > > Siara Systems, [email protected]
> > >
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